1. Field of the Invention
This invention relates to converting a format of image data, for example from block data to raster data. In particular, this invention is related to converting an order of pixel data supplied in a predetermined format.
2. Description of the Related Art
A conventional compression method that utilizes a Discrete Cosine Transform (DCT), such as JPEG, has been used for compressing and decompressing digital image data. In compression, it is usually necessary to convert raster form image data into block form image data. In decompression, it is usually necessary to convert block form image data into raster form image data.
FIG. 7 explains the operation of such a conventional compression/decompression method. In FIG. 7, image capturing of extracting part 601 comprises optical lenses, image pick-up elements, signal processing circuits and so on. DRAM 602 stores image data. The image data stored in DRAM 602 is used by image display part 603, such as CRT and LCD, for displaying an image. Therefore, the image data is stored in a raster display form for image display part 603. Buffer memory 604 is used for converting raster form image data into block form image data or for converting block form image data into raster form image data. Compressor/decompressor 605 performs compression/decompression using a method such as DCT. The image data compressed by compressor/decompressor 605 is stored in storage medium 606.
When image data stored in storage medium 606 is decompressed, the compressed image data is read out from storage medium 606, supplied to compressor/decompressor 605, and expanded. The decompressed image data is output to and written into buffer memory 604.
Consider a case where a number of pixels in a horizontal direction of uncompressed image data (i.e., original image data) is H. It is necessary for buffer memory 604 to have a holding capacity for data for at least 8.times.H pixels, assuming that the block form data has 8 rows. During decompression, the image data is written into buffer memory 604 from compressor/decompressor 605 in units of blocks. When the image data is read out of buffer memory 604 and supplied to DRAM 602, the block form image data is converted to raster form image data. As a result, the decompressed image data shown in FIG. 8 is written into DRAM 602 and an image is displayed on image display part 603, as shown in FIG. 8.
On the other hand, when image data from image capturing part 601 is compressed, the image data is written in DRAM 602 in raster form. The image data in DRAM 602 is divided into groups of 8 rows by H pixels of image data and transferred to buffer memory 604. Thus, 8.times.H pixels of image data are written into buffer memory 604.
Compressor/decompressor 605 reads out the image data from buffer memory 604 in units of, for example, 8.times.8 pixel blocks and compresses them. In this way, the image data compressed by compressor/decompressor 605 is stored in storage medium 606.
However, according to the above conventional method, the maximum value for it is given by the following equation: ##EQU1##
Here, m is the number of bits which represents one pixel.
Therefore, a maximum horizontal size of an image that can be processed by compressor/decompressor 605 depends on the capacity of buffer memory 604.